The present invention relates to a document density detector apparatus used in an image forming apparatus, such as an analogue or digital electrophotographic copying apparatus or a facsimile, which has a document reading mechanism and is adapted to process the read document information, thereby to form an image. More particularly, the present invention relates to a document density detector apparatus used in a image forming apparatus having enlarging and/or reducing functions for changing the document image at the desired magnification. More specifically, the present invention relates to a document density detector apparatus used in an image forming apparatus which includes (i) an optical pass for guiding light from the document to a light receiving body and (ii) a focal lens movable in the optical pass toward the document or toward the light receiving body and in which, by moving the focal lens in the optical pass, the document image formed on the light receiving body may be enlarged or reduced at the desired magnification.
The following description will discuss an analogue electrophotographic copying apparatus as an example of the image forming apparatus above-mentioned.
There is known an analogue electrophotographic copying apparatus in which the document is pre-illuminated and scanned, and the light reflected from the document is detected in intensity by a photosensor, according to the output of which the density of the document is automatically adjusted (For example, Japanese Unexamined Patent Publication No. 60-117264/1985 or Japanese Unexamined Patent Publication No. 61-63864/1986).
FIG. 18 and FIG. 19 are schematic perspective and plan views, respectively, illustrating a propagation area of the light which illuminates the document and is reflected therefrom and to which the photoreceptor is exposed (hereinafter referred to as light propagation area). Light projected from a light source (not shown) is reflected from a document 2, and the light thus reflected is collected by a focal lens 5. The light thus collected then illuminats a photoreceptor 3. A light sensor 1 such as a photosensor or the like is disposed outside of the light propagation area 4 of the light given from the document 2 to the photoreceptor 3 such that the light sensor 1 does not intercept the light to be given to the photoreceptor 3. The light sensor 1 is attached to a lens unit (not shown), and adapted to detect the light reflected from the document 2 substantially at the center thereof.
For the electrophotographic copying apparatus having enlarging and/or reducing functions, the focal lens 5 is movable in the optical pass toward the document 2 or toward the photoreceptor 3. For equal-magnification copy, the focal lens 5 is located in a position shown by solid lines in FIG. 19. For enlargement copy, the focal lens 5 is moved to a position 5a upstream of the position above-mentioned, as shown by a two dot chain line, while for reduction copy, the focal lens 5 is moved to a position 5b downstream of the position above-mentioned.
As mentioned earlier, the light sensor 1 is attached to the lens unit, and the position thereof is fixed regardless of the movement of the focal lens 5.
When the light sensor 1 and the focal lens 5 are arranged as above-mentioned and the focal lens 5 is movable in the optical pass, the following problems may be encountered:
(A) There is the liklihood that the focal lens 5 moved to the upstream position 5a for enlargment copy intercepts a portion of an optical pass 6 of the light incident upon the light sensor 1 (In FIG. 19, the light is intercepted in the hatched area).
If the density of a document is detected under such a condition, the document density thus detected is erroneously judged as higher than the actual one, since the amount of light detected by the light sensor 1 is decreased in an amount of the intercepted incident light.
(B) To solve the problem (A) above-mentioned, the light sensor 1 may be kept away from the focal lens 5. This not only makes it difficult to adjust the amount of light received by the light sensor 1, but also requires to increase in size the lens unit (not shown) to which the light sensor 1 is attached. Further, if the light sensor 1 is moved upstream, this newly presents the problem that the light sensor 1 prevents a light reflector (not shown) forming the optical pass, from moving.
(C) To solve the problem (A) above-mentioned, it may be also proposed to arrange the light sensor 1 to be movable together with the focal lens 5. However, since the light sensor 1 is connected to an electric wire, such a proposal involves another problem in view of the safety standards of the apparatus.
It is an object of the present invention to provide a document density detector apparatus used in an image forming apparatus, capable of accurately detecting the density of a document without interception of the light given to the light sensor 1 by the focal lens 5 at the time of detection of the document density.
It is another object of the present invention to provide a document density detector apparatus used in an image forming apparatus, capable of accurately detecting the density of a document even though the focal lens 5 intercepts a portion of the light incident on the light sensor 1 at the time of detection of the document density.